For those who don't know, sunspider is a Java based benchmark. This is a benchmark that Chrome has always dominated in the past.
Firefox 4.0.1 Sunspider Test 5/6/2011
Google Chrome v12.0.742.21 Sunspider Test 5/6/2011
Aurora 5.0 Alpha 2 Sunspider Test 5/6/2011
This version of Aurora is slated to be the 5th version of Firefox. After Mozilla Firefox 4's very long beta process Mozilla decided to create an additional channel for faster production process. Previous builds had the Nightly build and the final release. With the start of Firefox 5's production Mozilla has created a new channel is a mid point between nightly and final. The 4 channels are now Nightly, Aurora, Beta, and Firefox/final release. Nightly releases are expected to be full of bugs and problems. They are compiled using the code that was created earlier that day, so it is not fully tested. Aurora is the mid point, it's compiled and most bugs are worked out prior to release into this channel. It is also updated over time by Mozilla with the latest versions of the more stable builds.
There you go... This should shed more light on this subject.
Note: I didn't test IE9.